Cathode ray tubes



April 3, 1956 l. J. P. JAMES ET AL 2,740,918

CATHODE RAY TUBES FJ' led Dec. l5, 1951 2 Sheets-Sheet l ZOOM f TRANSLUCENT METAL SIGNAL. ELECTRODE.

GLASS END FIG' 5' WALL OF TUBE.

\M|CA OR GLASS -TARGET 2 l LECTRIC.

April 3, 1956 x. J. P. -IlAMEs ET AL CATHODE RAY TUBES 2 Sheets--SheeI 2 Fjled Dec. l5, 1951 Y j/l/ t//J/fl f /nvenvrs /VANHOE JOHN PENFOUND JAMES PETER FREDERIC THOMAS CRYR STILLVVELL CATHODE RAY TUBES Ivanhoe John Penfound James, South Ealing, London, and Peter Frederic Thomas Cryer Stillwell, Hadden ham, Aylesbury, England, assignors to Electric & Musical Industries Limited, Hayes, England, a company of Great Britain Application December 1S, 1951, Serial No. 261,804

Claims priority, application Great Britain December 16, 1950 9 Claims. (Cl. 315-10) This invention relates to charge storage pick-up tubes and in particular to such tubes employed as television pick-up tubes of the type wherein an electron beam is arranged to scan a target electrode on which a charge image is formed corresponding to the scene to be televised and in which picture signals are generatedin a signal electrode capacitatively associated with the target electrode. In such tubes it is customary to provide a wall anode for the purpose of producing a uniform accelerating field for the electrons, such wall anode being for example in the form of a conducting coating applied to the inner surface of the envelope of the tube. In order to etect the desired deiiections of the beam it is customary to provide deecting coils outside the envelope, to which coils a sawtooth current is fed. Ideally, signals should be generated only during the forward deflections of the beam, but in practice it is found that spurious signals are picked up by the signal electrode during the yback periods by reason of the high voltage pulses generated in the deflecting coils upon the collapse of current therein. In some tubes the spurious signals have a value which is much greater than the `Signal corresponding to peak White. The spurious signals are objectionable because they tend/to overload the amplifiers to which the output of the pickup tube is fed, and also were a device is provided which determines the absolute level of the television signals with reference to the level corresponding vto black during each line of the scanning raster, theV spurious signalsI interfere with the operation of said device.- f

The object of the present invention is to` provide a tube of the type referred to in which the pick-up of spurious signals by the signed plate is reduced.

It is Athought that these spurious signals are induced in said anode and since the anode is directly facing the signal electrode they are then transferred to the signal electrode via the capacity existing between the anode and the signal electrode.

According to the invention, there is provided an electric circuit arrangement including a pick-up tube comprising an envelope having therein a kcharge storage target electrode, a signal electrode associated with said storage electrode, an electron gun and an anode electrode system extending between said gun and said target electrode, said anode electrode system comprising first and second spaced annular parts, electro-magnetic deflecting means surrounding only the first anode part remote from said signal electrode, means for maintaining said anode parts atV a D. C. potential and means for maintaining said secondanode part which is nearer to said signal electrode at earth potential for high frequencies, whereby high frequency voltage fluctuations induced in said rstmentioned anode part from said deecting means are substantially prevented from reaching said second anode part so that the pickup of spurious high frequency signals bysaid signal electrode from said anode vsystemk issubstantially reduced. v I t 2,740,918 Patented Apr. 3, 1956 In order that the invention may be more fully understood it will now be described'with reference to the accompanying drawings, in which:

Figure 1 illustrates a typical form of pick-up tube, provided with a two part anode according to the invention, and

Figures 2, 3 and 4rillustrate various ways in which electrical connection may be made to the anodes.

Figure 5 is an enlarged view of the target electrode of the tubesshown in Figs. le4.

Referring to Figure l the glass envelope of the pickup tube compri-ses a tubular body portion 1 and an enlarged tubular end portion 2 which are joined together by means of a shoulder 3, a neck 4 being joined to the other end of the body portion 1 by means of a second shoulder 5. The end of the enlarged end portion 2 is closed by sealing to it a flat glass window 6 and the end of the neck 4 sealed off by means of a contact carrying base 7. At one end of the tubeV within the neck 4, there is provided an electron gun 8 of conventional form for generating a beam of electrons land at the other end Within the tubular portion 2 there is provided a charge storage target electrode 9 arranged close to the inner surface of the window 6. The target electrode 9 may suitably comprise a thin sheet of mica or glass having onthe side facing the electron gun 8 a mosaic of photo-electric elements 9a, its reverse side being coated with a semi-transparent continuous layer of metal 9b which in operation of the tube forms the signal electrode.

Along the length of the internal wall of the tubular body` portion 1 there is disposed au anode electrode system comprising first and second spaced annular parts 10 and 13, formed of graphite or metal. in the formation of the anode part 1t) care is taken to ensure that the coating does not extend over the surface of the shoulder 3 although if desired it may extend over the shoulder 5 and part way along the neck 4. Contact is made to anode part 10 by means of an internal connecting lead 11 and contact pin 12. The anode part 13 is arranged in the enlarged tubular portion 2 and extends away from the shoulder 3 along the internal wall in the direction of the target 9. Contact to this anode part 13 is made by means of a lead 14 sealed into the shoulder 3.

The tube is also provided with a decelerating electrode 15 formed as a coating on the wall of the envelope arranged so that it is close to the target 9, and with an ion trapping mesh 16 which may be directly connected to the anode part 13 inside the tube.

When such ak tube is mounted in a camera, it is surrounded by deecting coils 17 and a longitudinal focussing coil 18 mounted on a non-magnetic metal supporting cylinder 19 the cylinder 19 being maintained at earth potential.

The tube above described is similar to a conventional tube adapted to be operated with a lo-w velocity beam and operates in the same manner, but has an essential difference in that, the anode system comprisesl first and second spaced annular parts 10 and 13, the anode part 10 extending over the inner surface of the body portion 1 of the envelope, and the part 13 extending over part of the internal wall of the enlargedend portion 2, the part of the anode which would normally extend over the shoulder 3 of the envelope being omitted. In the operation of this type of tube spurious signals are introduced into the part 10 of the wall anode system during the yback periods of `the scanning sequence by reason of the high voltage pulses generated in the deflecting coils upon the collapse of the current therein.

By omitting theanode coating from the shoulder 3 the direct capacity between the anode and the signal plate is substantially .reduced whereby introduction of spurious signals into the video signal is minimised. The direct capacity between the portion of the anode part which in some instances extends over the shoulder 5 is very small and has noty been found to seriously interfere with the operation of the tube. A substantial reduction inl pick-up of spurious Signals from the anode system 10 and V13 is achieved according to the invention by connecting the anode parts 10 and 13 in an as sociated electrical circuit so that'they are independent of each other as far as high frequency currents are concerned and are separately decoupled to earth. As shown, each anode part 10 and 13 is connected to a D. C. supply indicated by the positive sign and to earth through the condensers 23 and 23a.

There are a number of other ways in which the anode parts 10 and 13 may be connected to a D. C. supply whereby separate earthing connections may be made thereto and typical examples are provided in the examples shown in Figures 2, 3 and 4. In each of these figures parts, the same as those described in connection with Figure 1, will be given the same reference numerals.

Referring now to Figure 2 the anode parts 10 and 13 are joined together as far-as D. C. is concerned via a re-y sistance 2i). This resistance 20 may be of the order of 50,000 ohms and may conveniently be provided by bridging the anode parts lll-and 13 across the shoulder 3 with a thin strip of high resistance material. gure the resistance 'coating is shown on the inside of the tube and such an arrangement simpliiies the construction but it is equally possible to seal separate contacts from the anode part 10 and anode part 13 through the shoulder 3 so that they are spacedl apart and then bridge these contacts by means of a resistance provided on the exterior of the tube. It will be understood that since the tube disclosed, operates with a low beam current the D. C. potential drop across the resistance 20 will be very small and therefore the two anode parts will have approximately the same potential. A suitable positive D. C. potential for anode parts 10 and 13 is 200 volts. The resistance 20 will provide a very high impedance to high frequency currents and therefore there is no danger of transferring the high frequency component o-f the anode part 10 to the anode part 13. l

To minimise the effect on the target 9 of spurious sig-l nals which may be introduced into the anode part 13, this anode part 13 is decoupled to earth via a condenser as in Figure 1 and this condenser is formed by coating, the external portion 2 of the envelope over the area occupied by the anode part 13 with metal or graphite as illustrated at 21. The envelope 1 forms the dielectric of the condenser and the coating 21 is connected to earth by contacting a spring strip. 22A secured to the metal cylinder19, supporting the focus vcoil 18.

The D. C. supply tothe anode part 10 .is made by connecting the contact pin 12at the gun end of the tube to the positive terminal of a suitable source of supply and is in turn decoupled to earth for high frequencies via a condenser 23.

In the example shown in Figure 3 the anode part 13 is provided with a contact 13a whichprojects through the wall of the tube at any convenient position along the wall portion 2 or shoulder 3 and from this contact a strip conductor 24 is provided which extends along the outer wall of the tube to the front edge of the glass window 6. When the tube is inserted into a camera casing it may be arranged that the conductor 24 engages with a contact whereby it is connected to the positive terminal of a D. C. supply and, isfdecoupled as regards high frequency currents by means'of a condenser 25 one plate of which is connected to; earth. Theiother anode part 10 is connectedva pine12' and decoupledto earth byv means of condenser 23 inthe same manner as. the tube described with reference to Figure-'2'. In order Vto'preyent In this the stray eld from scanning coils 17 from being. induced into the anode part 13, an electrostatic screen 26 conforming to the shape of the tube may be arranged over the shoulder 3 so that it extends partly over the anode part 10 and completely over the anode part 13. The screen 26 is maintained at earth potential and this may be achieved lby ensuring that it makes Contact with the cylinder 19 as by means of a springv strip 27.

In the arrangement shown in Figure 4 the tube is constructed exactly as described with reference to Figure 1 and the D. C. supply to the anode part 13 is obtained by connecting a line wire lead 28 from the contact 14' along the wall of the tube to the contact 12. Since this lead passes adjacent to the scanning field it may be found necessary to enclose this wire in an electrostatic screen maintained at earth potential.

In this construction although the two anodes parts 10 and 13 are connected to the same point as far as D. C. is concerned they are at different points as regards high frequency currents because of the inductance of the connecting strip 11 joining the anode part 10 and the pin 12. Decoupling for both the anode parts 10 and 13 is effected by a single condenser 23.

Itwill be understood that an electrostatic screen similar to 26 shown in Figure 3 may be employed with. the tube constructions shown in Figures 1, 2 and 4 and further that the tube constructiony shown in Figures 2 and 3 may also include ion trapping mesh 16 as shown in Figures 1 and 4.

The inclusion of an ion trapping mesh 16 forms an electrostatic screen between the target electrode 9 and the anode part 10 and thereby assists in the reduction of ode electrode and the signal plate with tubes having a' two-part'anode electrode system is shown by the following experimental results. In a tube having a wall anode extending over the body and the enlarged end portion and over the shoulder of the envelope, and provided with an ion trap mesh connected to the wall anode, and having l a deceleratingI electrode connected to ground via a condenser, it was found that the capacity between the wall anode and the signal electrode was 1.0 nnF. In a tube which was substantially similar except that the wall anodev was omitted from the shoulder and the mesh wasconnected to the minor part of the wall anode, the capacityv was less than 0.002 ,4r/LF. In a similar tube tothe lastmentioned one but with the mesh omitted the capacity was 0.018 WLF. When however the two parts of the wall anode were connected together via a low'resistance the capacity rose to 0.74 MLP.

What we claim is:

1. An electric circuit arrangement including a pickup tube comprisingr an envelope having therein a charge storage target electrode, a signal electrode associated with said; storage electrode, an electron gun, an anode electrode system extending between said gun and said target electrode, said anode electrode system comprising rst and second spaced annular parts, electro-magnetic deflecting means surrounding only the rst anode part remote from said signal electrode, means for maintaining said anode'parts at a D. C. potential, and means for maintaining said second anode part which is nearer to said` signal electrode at earth potential for high frequencies',`

are substantiallyl prevented from reaching saidl second anode part so that the pick-up of spurious highf frequencysignals by said signal electrode from said anode system is substantially reduced.

2. An electric circuit arrangement according to claim l, including a contact element passing through said envelope and the space between said anode parts, a contact element at the gun end of said tube, and a lead extending along the exterior of said envelope connected to said two contact elements, said iirst anode part being also connected to said contact element at the gun end of said tube.

3. An electric circuit arrangement according to claim 1 including a grid electrode extending transversely across said tube and connected directly to said second anode part.

4. An electric circuit arrangement including a pick-up tube comprising an envelope having a shoulder portion connecting one smaller portion of the envelope to an enlarged portion thereof, a charge storage target electrode in the enlarged portion of said envelope, a signal electrode associated with said storage electrode, an electron gun in the smaller portion of said envelope, an anode electrode system extending between said" gun and said target electrode, said anode electrode system comprising rst and second spaced annular parts with the iirst anode part in the smaller portion of the envelope and terminating at a point coinciding substantially with the commencement of said shoulder and the second anode part arranged in said enlarged portion, electro-magnetic deflection means sur rounding only the rst anode part in the smaller portion of said envelope, means for maintaining said anode sys tem parts at a D. C. potential, and means for maintaining said second anode part which is nearer to said signal electrode at earth potential for high frequencies, whereby high frequency voltage fluctuations induced in said rstmentioned anode part from said deiiecting means are substantially prevented from reaching said second anode part so that the pickup of spurious high frequency signals by said signal electrode from said anode system is substantially reduced.

5. An electric circuit arrangement according to claim 4 including an electric contact element passing through said shoulder and connected to said second anode part arranged in said enlarged portion, and separate connections to said rst anode part and to said contact element connecting said parts to a source of D. C. potential and separate capacities for maintaining said separate anode parts at earth potential for high frequencies.

6.- An electric circuit arrangement including a pick-up tube comprising an envelope having therein a charge storage target electrode, a signal electrode associated with said storage electrode, an electron gun, an anode electrode system extending between said gun and said target electrode, said anode electrode system comprising rst and second spaced annular parts, electro-magnetic deilecting means surrounding only the first anode part remote from said signal electrode, a high resistance interconnecting said two anode parts, means for connecting said iirst anode part to a source of D. C. potential, and means for maintaining said second anode part which is nearer to said signal electrode at earth potential for high frequencies, whereby high frequency voltage iluctuations induced in said rst anode part from said detiecting means are substantially prevented from reaching said second anode part so that the pick-up of spurious high frequency signals by said signal electrode from said anode system is substantially reduced.

7. An electric circuit arrangement including a pick. up tube comprising an envelope having therein a charge storage target electrode, a signal electrode connected with said target electrode, an electron gun, an anode electrode system extending between said gun and said target electrode, said anode electrode system comprising rst and second spaced annular. parts, electro-magnetic deflecting means surrounding only the irst anode part remote from said signal electrode, means for maintaining said anode parts at a D. C. potential, and means for maintaining said second anode part which is nearer to said signal electrode at earth potential for high frequencies, said last named means including a capacity formed by said second anode part, the dielectric of said envelope and a conductive covering surrounding said second anode part on the exterior of said envelope, whereby high frequency voltage fluctuations induced in said first-mentioned anode part from said deecting means are substantially prevented from reaching said second anode part so that the pick up of spurious high frequency signals by said signal electrodc from said anode system is substantially reduced.

S. An electric circuit arrangement according to claim 7 including a conductive sheath enclosing said tube, and a contact element connecting said conducting sheath to said conductive covering.

9. An electric circuit arrangement including a pick-up tube comprising an envelope having therein a charge storage target electrode, a signal electrode associated with said storage electrode, an electron gun, an anode electrode system extending between said gun and said tar get electrode, said anode electrode system comprising rst and second spaced annular parts, electro-magnetic deflecting means surrounding only the first anode part remote from said signal electrode, means for maintaining said irst anode part at a D. C. potential, and separate means for maintaining said second anode part at a D. C. potential, said separate means including a connection from said second anode part through said envelope connected to a conductor provided on the exterior of said envelope to a position near to said signal electrode and a contact to engage said conductor near to said signal electrode, said contact being connected to earth through a condenser and to a source of D. C. potential, whereby high frequency voltage fluctuations induced in said first anode part from said deflecting means are substantially prevented from reaching said second anode part so that the pick-up of spurious high frequency signals by said signal electrode from said anode system is substantially reduced.

References Cited in the le of this patent UNITED STATES PATENTS 2,322,807 Iams Iune`29, 1943 2,404,046 Flory et al. July 16, 1946 2,548,118 Morton et al. Apr. 10, 1951 2,555,091 Lubszynski May 29, 1951 2,579,351 Weltner Dec. 18, 1951 

